DLCs and/or a sequel would definitely be cool but I have doubts that it will be in the form of new SP missions with the original four Ghostbusters. I think we were VERY lucky to get the original actors to lend their voices to this game and I highly doubt we'll see them do it again, especially with Bill Murray. I would love to be wrong but I doubt I am. Of course, a MP DLC patch for the PC version would be most welcome.
IMO any kind of a sequel we'll get will involve new characters in a new setting. They certainly set it up for “franchising” a sequel game to a new city. Maybe if we're lucky we'll get Harold Ramis or Dan Akroyd to join in and we can at least get one or two familiar characters but I'm not going to hold my breath for it.